1. Field of the Invention
This invention relates to differential phase-shift keying (DPSK) in telecommunication. More specifically, it relates to methods in DPSK for converting a phase-keyed signal to an intensity-keyed signal.
2. Description of the Prior Art
Phase-shift keying (PSK) is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal (the carrier wave). Any digital modulation scheme uses a finite number of distinct signals to represent digital data. In the case of PSK, a finite number of phases is used. Each of these phases is assigned a unique pattern of binary bits. Usually, each phase encodes an equal number of bits. Each pattern of bits forms the symbol that is represented by the particular phase. The demodulator, which is designed specifically for the symbol-set used by the modulator, determines the phase of the received signal and maps it back to the symbol it represents, thus recovering the original data. This requires the receiver to be able to compare the phase of the received signal to a reference signal (such a system is termed coherent).
Alternatively, instead of using bit patterns to set the phase of the wave, the patterns are used to set changes in the phase of the signal. The demodulator then determines the changes in the phase of the received signal rather than the phase itself. Since this scheme depends on the difference between successive phases, it is termed differential phase-shift keying (DPSK). DPSK can be significantly simpler to implement than ordinary PSK because there is no need for the demodulator to have a copy of the reference signal to determine the exact phase of the received signal (i.e., it is a non-coherent scheme).
In telecommunication technology, differential phase-shift keying utilizes a decoding method in order to convert the phase-keyed signal to an intensity-keyed signal at the receiving end. The decoding method can be achieved by comparing the phase of two sequential bits. In principle, it splits the input signal beam into two channels with a small delay before recombining them. After the recombination, the beams from the two channels interfere constructively and destructively. The interference intensity is measured and becomes the intensity-keyed signal. To achieve this, one channel has an optical path longer than the other by a distance equivalent to the photon flight time of one bit. For instance, in a 40 Gbit-per-second system, one bit is equal to 25 ps and light travels 7.5 mm in that period. Thus, in this example, the optical path difference (OPD) between the two channels would be set at 7.5 mm.
The Mach-Zehnder type interferometer with a desired OPD between the two channels has been used for decoding purposes. Because of the properties of optical interference, a change in OPD can greatly affect interference intensity. Moreover, the optical path in each arm is much longer than its difference. Therefore, a sophisticated temperature control is required to maintain the optical path in each arm in order to assure that the change in the OPD is much less than a small fraction of one wavelength, e.g., about 10 nm. This is difficult and expensive to achieve, especially for an interferometer with a long optical path.
Copending U.S. application Ser. No. 11/360,959 and Ser. No. 11/485,653 describe various embodiments of novel Michelson-type interferometers used as DPSK demodulators to determine the changes in the phase of a received signal. In the demodulator, the input beam is split into two portions at the beam splitter. The two beams travel a different path and are returned by their corresponding reflector. Because the optical path lengths (OPLs) are different, the two returned beams have a time delay with respect to each other. The OPD of the system, the difference between the two OPLs, is designed to assure that the delay is approximately equal to the time delay of any two successive bits and is equal to the time interval multiplied by the speed of light.
These demodulators based on Michelson-type interferometers provided a significant improvement over the prior art; however, they still require a substantially perfect balance between the two arms of the interferometer in terms path length, polarization phase shift, and thermal compensation. Copending U.S. application Ser. No. 12/505,483 describes a Michelson configuration with a pseudo common-path delay-line design that materially simplifies the process of achieving and maintaining the required optical path difference in the two arms of the interferometer. This invention describes a true common-path demodulator based on a Sagnac interferometer configuration.